Abstract

Although there has been extensive research on the thermoelectric effect, there have only been some reports on the photo-thermoelectric effect in carbon nanotubes, i.e. the conversion of light to heat to electricity. A device capable of producing a thermoelectric voltage by light irradiation on a forest of aligned multi-walled carbon nanotubes has not yet been reported. The work presented in this thesis first outlines the growth conditions by which millimetre-long CNTs were grown by catalytic chemical vapour deposition (CVD). Two novel thermoelectric devices were fabricated based on two intrinsic properties of CNT forests. (1) Using the “Heat Trap” effect (light-induced heat localization), the first device induced a potential difference (few hundred μV) from low incident laser power. The temporal dynamics of the induced voltage were understood to be due to a competition of the temperature gradients within the device materials. A finite element analysis model was simulated the thermal and electrical characteristics. The temperature-dependent thermal conductivity of CNTs was derived based on the experimental induced voltage and was seen to fall-off with temperature, confirming a previously-suggested mechanism for the effect. (2) Since the thermal conductivity of CNTs can be 1-2 orders of magnitude smaller in the direction perpendicular to the nanotube axis, a second device was fabricated to achieve a higher temperature gradient. Under a few hundred mW of laser power, a few mV of potential difference was induced, indicating power conversion efficiencies in the 10ˉ⁵ % range. A finite element analysis model was created, which by using the experimentally-derived thermal conductivity from the previous device, predicted the induced voltage to within 10% at high laser powers. Calculation of the room temperature figure-of-merit ZT was low (10ˉ⁶ range), however no device optimization had been performed in these proof-of-concept prototypes. If CNT-based thermoelectric devices are to be used at higher temperatures, the three temperature-dependent material parameters enhance ZT and the efficiency. CNTs can be a promising material choice if cost and low toxicity are concerns, since the CVD process is fairly inexpensive and carbon-containing precursors are abundant. Moreover, CNTs have a high power-to-weight ratio and CNT forests are sparse.